It is known to brace the panels of rectangular sheet-metal air-ducts against inward bowing by the use of an adjustable-length rod (U.S. Pat. No. to Savage 3,557,838). But no brace is known which is especially adapted for use with soft fiberboard duct-panels and/or which braces such panels against buldging as well as against inward bowing. It is accordingly the principal object of this invention to provide such a brace. Other objects and advantages will become apparent as the following detailed description proceeds.